The present invention relates to a method of processing a semiconductor device made of silicon and a method of fabricating the device, and more particularly relates to suppressing the degradation of the device due to Staebler-Wronski (S-W) effects.
As disclosed by D. L. Staebler, C. R. Wronski in Applied Physics Letters, 31, No. 4, p. 292, 1977, it is known that the electrical conductivity of silicon when exposed to light (which will be herein referred to as xe2x80x9cphotoconductivityxe2x80x9d) and the electrical conductivity thereof in the absence of light (which will be herein referred to as xe2x80x9cdark conductivityxe2x80x9d) both decrease rapidly with time. This phenomenon is called xe2x80x9cS-W effect degradationsxe2x80x9d. The photoelectric conversion efficiency of a solar cell decreases as the electrical conductivity (i.e., photo-conductivity) of amorphous silicon decreases. Thus, it is important to minimize the decrease.
In view of this inevitable phenomenon, the following methods are employed to suppress the decrease in photoconductivity of amorphous silicon.
In a first method, SiD4 is used instead of silane gas (SH4) when an amorphous silicon film is formed by a chemical vapor deposition method (CVD). This method is taught by, for example, Koji Dairiki, Seiichi Suzuki, Akira Yamada, Makoto Konagai in Technical Digest of the International PVSEC-9. p. 373, 1996.
In a second method, SiF4 is used instead of silane gas during a CVD process. This method is taught by, for example, Y. Kuwano, M. Ohnisi, H. Nishiwaki, S. Tsuda, H. Shibuya, S. Nakano in Proceeding of 15th IEEE Photovoltaic Specialists Conference, p. 698.
In a third method, a pinpin structure with double pin junctions is formed to reduce the thickness of an i-layer between p- and n- layers and, thereby suppress the S-W effect degradation.
However, the above-mentioned methods have the following drawbacks.
In the first method, the cost increases because SiD4 is expensive. In addition, although the exposure-induced change of photoconductivity is suppressible to some degree, it is still impossible to completely eliminate the change of photoconductivity.
In the second method, when SiF4 is used, electrical discharge power needed for forming amorphous silicon is almost ten times as high as the power needed when silane gas is used. In addition, the resultant film is likely to have impurities mixed in and the properties of the film easily deteriorate. Furthermore, the film easily peals off.
In the third method, the fabricating cost to form the pinpin structure is much higher than usual, and yet the resultant device cannot eliminate the S-W effect degradation.
The S-W effect degradation phenomenon is also observable in polysilicon, microcrystalline silicon, and single crystalline silicon. It is known that microcrystalline silicon and polysilicon, for example, have a large number. of silicon dangling bonds especially in grain boundaries. Single crystalline silicon also has dangling bonds if the single crystalline silicon is amorphized as a result of ion implantation, for example, and cannot recover fully to its original state. Accordingly, if amorphous silicon, polysilicon, microcrystalline silicon or single crystalline silicon is used as a material for a photoelectric conversion device such as solar cell or optical sensor, the optical characteristics of the device such as the photoelectric conversion efficiency thereof might deteriorate.
An object of the present invention is providing methods of processing and fabricating a photoelectric conversion device having high conversion efficiency at a practical cost. In order to achieve this object, the present invention takes various measures to readily eliminate defects that cause S-W effect degradation in amorphous silicon, polysilicon, single crystalline silicon and microcrystalline silicon, for example.
A silicon photoelectric conversion device according to the present invention comprises: a silicon layer containing CN groups; and a conductor layer provided either over or under the silicon layer.
According to the present invention, defects such as dangling bonds, weak bonds and strained bonds in the silicon layer are replaced by CN groups. Thus, even if carriers have been activated upon exposure to light, the silicon layer has almost no defects that can be centers of recombination. As a result, a device such as solar cell, optical sensor and (electrophotographic) photoreceptor with good characteristics is realized. This is because the device includes the silicon layer with much improved properties (e.g., photoconductivity) that will hardly deteriorate with time even upon exposure to light.
In one embodiment of the present invention, the silicon layer is preferably made of at least one material selected from the group consisting of amorphous silicon, polysilicon, microcrystalline silicon and single crystalline silicon.
In another embodiment, the conductor layer may be a lower conductor electrode for a solar cell, and the device may further comprise an upper conductor electrode provided on the silicon layer. In such an embodiment, a solar cell with a photoelectric conversion efficiency, which will not decrease but can be kept sufficiently high even after exposure to light, can be obtained.
In still another embodiment, the device may further comprise a lower semiconductor layer interposed between the lower conductor electrode and the silicon layer. In such an embodiment, the absorption spectrum of the device can be broadened. Thus, the photoelectric conversion efficiency of the device further increases.
In this particular embodiment, the lower semiconductor layer may be made of at least one material selected from the group consisting of p- or n-microcrystalline silicon, p- or n-amorphous silicon, p- or n-polysilicon and p- or n-single crystalline silicon.
In yet another embodiment, the device preferably further comprises an upper semiconductor layer interposed between the upper conductor electrode and the silicon layer.
In this particular embodiment, the upper semiconductor layer may also be made of at least one material selected from the group consisting of p- or n-microcrystalline silicon, p- or n-amorphous silicon, p- or n-polysilicon and p- or n-single crystalline silicon.
In yet another embodiment, one of the lower and upper conductor electrodes may be made of a conductor which is transparent to sunlight, and the other electrode may be made of a conductor which is opaque to sunlight. Then, the absorption efficiency of sunlight increases.
In still another embodiment, the device may further comprise an insulator layer interposed between the silicon layer and the conductor layer. Then, the present invention is applicable to various kinds of devices such as a thin film transistor (TFT).
In this particular embodiment, the insulator layer is preferably made of at least one material selected from the group consisting of silicon dioxide (SiO2), silicon monoxide (SiO), trisilicon tetranitride (Si3N4), silicon oxynitride, titanium dioxide (TiO2), aluminum trioxide (Al2O3) and tungsten trioxide (WO3).
An inventive method of fabricating a silicon photoelectric conversion device comprises the steps of: a) forming a silicon layer over a substrate made of a conductor, a semiconductor or an insulator; and b) performing a cyano process that introduces cyano ions (CNxe2x88x92) into the silicon layer.
According to the present invention, cyano ions (CNxe2x88x92) enter the silicon layer. As a result, defects such as dangling bonds in the silicon layer are terminated with the cyano ions, and the weak bonds and strained bonds are replaced by CN groups. Thus, even if carriers have been activated upon exposure to light, the silicon layer has almost no defects that can be centers of recombination. As a result, a device, including a silicon layer with much improved properties (e.g., photoconductivity) that will hardly deteriorate with time even upon exposure to light, can be fabricated.
In one embodiment of the present invention, at least one layer selected from the group consisting of an amorphous silicon layer, a polysilicon layer, a microcrystalline silicon layer and a single crystalline silicon layer is preferably formed in the step a) as the silicon layer.
In this case, it is the simplest way that the substrate is entirely immersed in a solution containing cyano ions (CNxe2x88x92) in the step b).
In another embodiment, the substrate may be made of an insulator. The method may further comprise the step of forming a lower conductor electrode on the substrate before the step a) is performed. And in the step a), the silicon layer may be formed on the lower conductor electrode.
Instill another embodiment, the method may further comprise the step of forming an upper conductor electrode on the silicon layer after the step b) has been performed or before the step b) is performed.
In still another embodiment, the method may further comprise the step of forming a lower semiconductor layer over the substrate before the step a) is performed. And in the step a), the silicon layer may be formed on the lower semiconductor layer.
In this particular embodiment, the method may further comprise the step of forming a lower conductor electrode on the substrate before the step a) is performed. And in the step a), the lower semiconductor layer may be formed on the lower conductor electrode.
In an alternative embodiment, the method may further comprise the step of forming an upper conductor electrode on the silicon layer after the step b) has been performed or before the step b) is performed.
In yet another embodiment, the method may further comprise the step of forming an upper semiconductor layer on the silicon layer after the step a) has been performed and before the step b) is performed. And in the step b), a cyano process may be performed on the upper semiconductor layer.
In this case, the method may also further comprise the steps of forming the lower conductor electrode and the lower semi-conductor layer as described above.
In yet another embodiment, the method may further comprise the step of forming an insulator layer on the silicon layer after the step a) has been performed. Then, the present invention is applicable to various kinds of devices such as a TFT.
In this particular embodiment, the method may further comprise the step of forming a conductor electrode on the insulator layer after the step b) has been performed.
In yet another embodiment, the method may further comprise the step of exposing the silicon layer to light after the step a) has been performed and before the step b) is performed.
An inventive method of processing a silicon photoelectric conversion device comprises the steps of: a) preparing a substrate to be processed including a silicon layer; b) preparing a process solution containing cyano ions (CNxe2x88x92): and c) performing a cyano process for introducing the cyano ions (CNxe2x88x92) into the silicon layer of the substrate using the solution.
According to the present invention, CNxe2x88x92 ions act on defects in the silicon layer of the substrate as described above. Thus, various properties of silicon such as photoelectric conversion efficiency can be improved.
In one embodiment of the present invention, the substrate to be prepared in the step a) preferably includes, as the silicon layer, at least one layer selected from the group consisting of an amorphous silicon layer, a polysilicon layer, a microcrystalline silicon layer and a single crystalline silicon layer.
In this case, it is the simplest way that the substrate is immersed in the solution in the step c).
In yet another embodiment, the substrate may further include an upper semiconductor layer formed on the silicon layer. And in the step c), the cyano process may be performed on the upper semiconductor layer.
In still another embodiment,. even if the silicon layer of the substrate has been exposed to light before the step c) is performed, the silicon layer in which S-W degradation occurs can recover its photoconductivity.